var subtract = function(a, b) { return b - a; };

subFrom20 = partial(subtract,_,20);
subFrom20(5);//15


function partial(fn){
    var args = [].slice.call(arguments,1)//[_,20]
    return function(){
        var newArgs =args.concat([].slice.call(arguments))
        fn.apply(this,newArgs)
    }
}